Numerous current measuring devices and magnetometers have been developed based upon the Faraday effect. The Faraday effect causes the plane of polarization of a polarized beam of light passing through a transparent substance exhibiting the Faraday effect to rotate from the plane of polarization of the incident light by an amount proportional to the magnetic field passing through the substance parallel to the optical axis of the beam of light.
U.S. Pat. Nos. 3,324,393, 3,419,802, 3,502,978, 3,581,202, 3,590,374, 3,597,683, 3,605,013, 3,693,082, 3,708,747, 3,746,983, 3,978,334, 3,980,949, 4,070,620, 4,070,622, 4,112,367, 4,232,264, 4,243,936, 4,255,018, 4,348,587, 4,363,061, 4,370,612, 4,428,017, 4,516,073, 4,529,875, 4,531,092, 4,539,519, 4,539,521, 4,540,937, 4,563,639, 4,563,646, 4,564,754, 4,578,639, 4,581,579, 4,612,500, 4,613,811, 4,631,402, 4,683,421, and 4,698,497, disclose current or magnetic field sensors based upon the Faraday effect.
Magneto-optic materials exhibiting the Faraday effect have been developed and are commercially available which have a substrate of gadolinium gallium garnet which is coated with a layer of yttrium iron garnet. For example, the aforementioned material is available with a 45 micron thickness coating having substituted bismuth with a relatively large lattice constant from the Airtron Division of Litton Industries In the prior art the usage of these types of materials has been based upon a direct connection of an optical fiber to a wafer of the aforementioned material which functions to transmit a plane polarized beam of light emitting from the end of the optical fiber by means of internal reflection in the material. The reason that internal reflection is necessary to transmit the light is that the light exiting an optical fiber has substantial divergence. The rays of light emanating from the end of the optical fiber therefore have varying light paths which must be internally reflected in order for the light beam to traverse the material exhibiting the Faraday effect. See FIG. 2 of U.S. Pat. No. 4,112,367. Additionally, U.S. Pat. No. 4,560,932 discloses a Faraday effect material in which the light traverses the material by repeated reflections from a reflective layer 15. Additionally, U.S. Pat. No. 4,516,073, which is referred to above, discloses the direct connection of an optical fiber to the end of a layer of yttrium iron garnet. Although not specifically discussed in the '073 patent, the direct connection of the optical fiber to the yttrium iron garnet layer necessarily results in internal reflection from the outside surfaces of the layer because of the aforementioned divergence of light at the end of an optical fiber.